Lithium ion batteries were first commercialized in 1990, and soon thereafter drew tremendous attention from academic and industry interests due to the advantages such as high energy density and rapid charge/discharge capabilities in comparison to state of the battery technology at the time. In recent years, lithium ion battery technology has become the most popular power source for portable electronic devices. In addition, lithium ion batteries have found application in hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV). However, safety of lithium batteries continues to plague the technology. For example, secondary lithium-ion batteries are known to exhibit problems in shorting of the battery, elevated operating temperatures and overcharge, which can lead to dangerous situations such as overheating, fire, and explosion of the battery.
Overcharge occurs when electricity flow is forced through a cell when its capacity is already full. This is one of the most common factors that could lead to serious safety issues of lithium-ion batteries. Due to the manufacture processes, there is always a “weakest cell” in a battery pack (i.e. the cell with the lowest charging capability in a multi-cell battery pack). During charging, the weakest cell will reach full capacity prior to the other cells, but because the overall voltage of the battery is not high, the full capacity cell with not trigger the voltage monitor of the charger to read “full.” As a result, the weakest cell is put into an overcharge situation. Instead of being stored evenly across all electrodes in the battery pack, electricity will build up in, and increase the potential of, the cathode in the weakest cell, causing the potential to go beyond the electrochemical window of the electrolyte. In turn, this will cause reactions to occur such as oxidation of the electrolyte, leading up to and including explosion of the cell and battery pack.
Known methods to avoid the overcharge abuse in practice, include the use of electronic devices attached to each individual cell to monitor for overcharge, the use of overcharge protection compounds in each cell, and the use of redox shuttles in the electrolyte of the electrochemical cells.
A number of redox shuttle compounds are known. Generally, the redox shuttle molecule can be reversibly oxidized and reduced at a defined potential slightly higher than the end-of-charge potential of the positive electrode. This mechanism can protect the cell from overcharge by locking the potential of the positive electrode at the oxidation potential of the shuttle molecules.
For an ideal redox shuttle compound, there are at least three desirable properties. The first property is that it should have a reversible oxidation potential that is appropriate for the cathode material with which it is to be used. This means that the oxidation potential of the redox shuttle should be between 0.3V and 0.5V volts higher than the end-of-charge potential of the cathode. This will ensure that redox shuttle is accessed only overcharge potentials. The second property is that the redox shuttle should be electrochemically stabile or reversible. The stability and reversibility of the redox shuttle will determine how much overcharge protection is provided. The third property is that the redox shuttle is to have sufficient solubility in the electrolyte system in order to have an effective amount of the redox shuttle available.